Local Coordination Disorder and Defect Formation in Cadmium Zeolitic Imidazolate Framework Glasses

Cadmium-based zeolitic imidazolate glasses provide an attractive model system for investigating local coordination changes during vitrification. Here, we examine the structure of Cd(Im)2-x(bIm)x (Im = imidazolate, bIm = benzimidazolate) in its as-synthesized and glassy states using solid-state NMR spectroscopy, differential scanning calorimetry, electron paramagnetic resonance, and complementary techniques. Increasing bIm content reduces crystallinity and suppresses recrystallization, promoting glass formation. While 13C and 1H NMR spectra indicate largely preserved linker environments, broadening of 15N and 113Cd resonances reveals increased local disorder. 113Cd{1H} REDOR measurements show that the average 113Cd-1H dipolar interaction is largely preserved upon vitrification. Thus, the predominant local Cd coordination environment is retained, although minor coordination defects cannot be excluded. EPR reveals thermally induced paramagnetic centers, whose microscopic origin remains unassigned and may include coordination-related defects and thermal decomposition. Variable-temperature 1H NMR further reveals dynamic changes across the glass-transition region. Overall, ligand chemistry influences melting, recrystallization, and glass formation in hybrid coordination networks. Vitrification combines the preservation of the predominant local Cd environment with an increase in local geometric disorder and the formation of thermally induced paramagnetic centers.